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The Specificity of Lipoxygenase-Catalyzed
Lipid Peroxidation and the Effects of Radical-
Scavenging Antioxidants

Abstract

The oxidation of low density lipoprotein (LDL) by
lipoxygenase has been implicated in the pathogenesis
of atherosclerosis. It has been known that
lipoxygenasemediated lipid peroxidation proceeds
in general via regio, stereo and enantiospecific
mechanisms, but that it is sometimes accompanied
by a share of random hydroperoxides as side reaction
products. In this study we investigated the oxidation
of various substrates (linoleic acid, methyl linoleate,
phosphatidylcholine, isolated LDL, and human plasma)
by the arachidonate 15-lipoxygenases from rabbit
reticulocytes and soybeans aiming at elucidating
the effects of substrate, lipoxygenase and reaction
milieu on the contribution and mechanism of random
oxidation and also the effect of antioxidant. The specific
character of the rabbit 15-lipoxygenase reaction
was confirmed under all conditions employed here.
However, the specificity by soybean lipoxygenase
was markedly dependent on the conditions. When
phosphatidylcholine liposomes and LDL were oxygenated
by soybean lipoxygenase, the product pattern
was found to be exclusively regio, stereo, and
enantiorandom. When free linoleic acid was incorporated
into PC liposomes and oxidized by soybean
lipoxygenase, the free acid was specifically oxygenated,
whereas esterified linoleate gave random
oxidation products exclusively. Radicalscavenging
antioxidants such as αtocopherol, ascorbic acid
and 2-carboxy-2,5,7,8-tetramethyl-6-chromanol selectively
inhibited the random oxidation but did not influence
specific product formation. It is assumed that
the random reaction products originate from free radical intermediates, which have escaped the active
site of the enzyme and thus may be accessible to radical
scavengers. These data indicate that the specificity
of lipoxygenasecatalyzed lipid oxidation and
the inhibitory effects of antioxidants depend on the
physicochemical state of the substrate and type of
lipoxygenase and that they may change completely
depending on the conditions.

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